The present invention relates generally to coating a substrate or moving web with a coating fluid, and more particularly to a method and apparatus for coating a moving web by conducting the web to a metered source of coating fluid and urging a pliable smoothing film against the web by means of controllable pressure generating means, such as a variable tension force web or an inflatable membrane, the pressure generating means being adjacent to and downstream of the position at which the coating fluid is metered onto the web and/or the smoothing film.
There exists a great number of means and apparatus for coating liquids onto moving substrates. For instance, doctor knives may be employed to smooth and control the coating onto the substrate. Rollers may be employed to apply coatings either singularly, i.e., instances in which the roller passes through a reservoir of coating liquid and conducts the liquid directly to the substrate, or in combination such as in instances in which the substrate is passed between the nip of adjacent rollers to control coating. Coatings may be extruded in quite thin layers directly onto the substrate. The substrate may be dipped into a reservoir of coating liquid, either free-running as a web, or while being conducted around a roller, and air brushes or resilient wipers may be employed to remove the excess coating material from the substrate.
An early example of one wiping means is found in U.S. Pat. No. 62,044, issued Feb. 12, 1867. In this patent, a static cloth is stretched partially around a roller to generate a wiping action for bronze powder.
More recent U.S. Pat. No. 3,688,738, issued Sept. 5, 1972, discloses a coating means in which a substrate in the form of a web is conducted around a roller which is immersed in a coating fluid. A wiping film, which does not extend beyond the roller, is employed to remove excess coating fluid from the substrate and return it to the reservoir. In certain embodiments, an additional static film is employed to bear against the wiping membrane. However, it is generally not desirable to coat to the edge of the substrate. Excess coating material is flung from the edge as the web advances. Also, it is often useful to provide an uncoated margin. Thus, the simple apparatus of U.S. Pat. No. 3,688,738 is rather restricted as to the nature of the coatings produced and speed of operation. U.S. Pat. No. 3,352,706 is another example of a coating method in which excess coating material is applied to a web and a squeegee, which may be a flexible piece of plastic or rubber backed by resilient members such as stiffer sheets of plastic or rubber, is employed to remove the excess coating liquid from the web. Again, no provision is made for other than complete coating of the web with excess coating liquid. The patent is rather specific as to the need for a run-off path for the material.
Devices and methods for coating both sides of a web in essence utilize the above-mentioned concepts. For instance, according to U.S. Pat. No. 4,076,864, a web is guided through a bath of coating liquid with excess coating liquid removed by a doctor blade adjacent a backup roller. U.S. Pat. Nos. 3,575,134 and 3,908,590 are further examples of devices for coating both sides of a web, such as paper.